Originally Posted by
Peter Laidler
That 20T mark is the military proof, done at the factory and not a commercial proof. This question was raised in a technical instruction because some L39/42 parts/bodies were marked 19T and later we got bolts and boltheads through marked 20T! (see bottom right photo of body marked 19T and bolt marked 19T AND 20T) Apparently it was the 'same' but done to a different method under instructions from Cold Meese (the Military proofing authority centre). That's as I recall it and it might not be strictly correct - so forgive me - but it was something like that. So far as Armourers were concerned, it cleared the air and we could use both markings. Maybe Ensci who was involved in the 19T/20T project could comment